1. Field of Invention
The present invention relates to light recycling within, for example, projection systems. The invention is more particularly related to light recycling in multi-channel projections systems such as for 3D projections.
2. Discussion of Background
Various methods are in place for 3D stereoscopic projection, including Anaglyph, Linear Polarization, Circular Polarization, Shutter Glasses, and Spectral Separation. Anaglyph is the oldest technology, and provides left/right eye separation by filtering the light through a two color filter, commonly red for one eye, and cyan for the other eye. At the projector, the left eye image is (commonly) filtered through a red filter, and the right image filtered through a cyan filter. The eyewear consists of a red filter for the left eye, and a cyan filter for the right eye. This method works best for black and white original images, and is not well suited for color images.
Linear Polarization 3D provides separation at the projector by filtering the left eye through a linear polarizer (commonly) oriented vertically and filtering the right eye image through a linear polarizer oriented horizontally. The eyewear consists of a vertically oriented linear polarizer for the left eye and a horizontally oriented polarizer for the right eye. The projection screen must be of the polarization preserving type, commonly referred to as a “silver screen” because of its distinctive color. Linear Polarization allows a full color image to be displayed with little color distortion. It has several problems, these include the need for a silver screen which is expensive, fragile, and not uniform. Another problem is that the viewer must keep his head oriented vertically to avoid crosstalk from one eye to another.
Circular Polarization 3D was invented to address the problem of requiring the viewer to keep his head oriented vertically. Circular Polarization provides separation at the projector by filtering the left eye image through a (commonly) left handed circular polarizer, and filtering the right eye image through a right handed circular polarizer. The eyewear consists of a left handed circular polarizer for the left eye and a right handed circular polarizer for the right eye. A silver screen is also needed for this approach.
Shutter Glasses provides separation by multiplexing the left and right images in time. A filter for separation at the projector is not required. The eyewear consists of active glasses that electronically shutter the lens in synchrony with the projector frame rate. The left eye image is first displayed, followed by the right eye image etc. Since having a direct wired connection to the Glasses in a theatre is impractical, a wireless or infrared signaling method is used to provide a timing reference for the left/right eye shuttering.
Spectral Separation provides separation at the projector by filtering the left and right eye spectrally. The system differs from anaglyph in that the filters for the left and right eye each pass a portion of the red, green, and blue spectrum, providing for a full color image. The band pass spectrum of the left eye filter is complementary to the band pass spectrum of the right eye filter. The eyewear consists of filters with the same general spectral characteristics as are used in the projector. While this method provides a full color image, it requires color compensation to make the colors in the left and right eye match the colors that were present in the original image, and there is a small reduction in the color gamut compared to the gamut of the projector.
The projectors themselves take on various forms, including LCD (liquid crystal display) projectors which usually contain three separate LCD glass panels, one each primary color component of a image to be projected. The LCD panels modulate the light and produces the image that is projected onto the screen.
DLP (“Digital Light Processing”) is a proprietary technology developed by Texas Instruments. The DLP chip is a reflective surface made up of thousands of tiny mirrors. In expensive DLP projectors, there are three separate DLP chips, one for each for the red, green, and blue channels. Typically, a prism separates light from a projection lamp into red, green, and blue colored light which then separately illuminate “red,” “green,” and “blue” DLP chips which modulate the primary colored lights according to the corresponding primary color components of an image signal. After modulation, the now modulated primary colored lights are recombined and projected onto a viewing screen.
Other, and less expensive, applications of DLP include projectors with a single DLP chip used in conjunction with a color wheel that consists of red, green, blue, and sometimes white (clear) filters. The color wheel spins between a projection lamp and the DLP chip—alternating the color of the light illuminating the chip. The alternating colors of the light illuminating the DLP chip are modulated and projected on a viewing screen in sequence. The sequentially modulated and projected lights then form a full color image when viewed.
The single chip DLP projector described has a lack of efficiency which has been partially addressed through light re-capture and the use of “scrolling color” across the DLP chip. Such techniques are described, for example, in D. Scott Dewald, “Sequential Color Recapture and Dynamic Filtering: A Method of Scrolling Color,” SID 00 DIGEST, and Dewald, U.S. patent application Ser. No. 10/028,023.